This invention relates to an improved wheel slip control system for electrically propelled vehicles.
In U.S. Pat. No. 3,930,189 issued Dec. 30, 1975 to Russell M. Smith and assigned to the General Electric Company, there is described a wheel slip system for use in a traction vehicle employing an alternating current supply and series-type direct current electric traction motors. A power rectifier converts the alternating current to direct current for the supply of normal direct current excitation to the armatures and fields of the motors. A current transformer has a primary winding serially connected in the alternating current path between the alternating current supply and the power rectifier. A secondary winding of the current transformer is connected to a secondary rectifier circuit which rectifies the current produced by the current transformer and couples it to field windings of the traction motors. The current from the current transformer is of such polarity that upon application to the field windings, the summation of the normal excitation current and the current transformer current maintains the total current in the field windings at a substantially constant value or at a value only slightly less than the value of current flowing prior to the occurrence of a wheel slip.
When a wheel slip occurs, the increased rotational velocity of the motor armature connected to the slipping wheel causes an increase in the counter electromotive force (CEMF) or voltage drop across that motor armature. Since the difference between the CEMF and the source voltage determines the magnitude of armature current, the increase in CEMF forces a reduction in armature current and, due to the series connection of armature and field windings, a similar reduction in field current. The reduction in field current tends to cause a reduction in CEMF which is compensated by an increase in armature velocity. This process of increasing armature velocity with attendant reduction in field current will continue until the motor torque is just balanced by the force exerted on the wheel. The wheel slip will be self-corrected by the motor if a sufficient frictional force between wheel and tractive surface is experienced. The system in the aforementioned U.S. Pat. No. 3,930,189 corrects a wheel slip by boosting field current so that motor armature CEMF is rapidly increased to a level which substantially reduces armature current. Since motor torque is proportional to the product of armature current and field current, the rapid reduction in armature current is such that the motor torque falls below the value necessary to maintain the wheel slip condition. Thus, the wheel is rapidly forced to re-adhere to the tractive surface.
Since it is not desirable to supply the additional current, i.e., boost current, from the current transformer when the wheels are not slipping, the secondary winding of the current transformer is maintained in a short circuited condition until a wheel slip occurs.
In the patented system the secondary winding is effectively short-circuited by semiconductor devices such as thyristors (SCR) or transistors. A wheel slip detector circuit supplies signals to maintain conduction of the short-circuiting semiconductor devices so that the devices are turned off (non-conducting) only upon detection of a wheel slip condition of the wheels that are being driven by the traction motor whose series field is coupled to the associated secondary winding. Accordingly, it can be seen that the wheel slip detector provides continuous gating signals which are interrupted upon the detection of a wheel slip condition. In the event of a failure of the wheel slip detector to supply gating signals, which failure may result from a component or power loss, the short-circuiting devices will not be gated into conduction and boost current will be supplied to the motor field windings. Such a result is undesirable since it will reduce the torque of the boosted motors and reduce the tractive effort of the driven wheels.